a. Field of the Invention
The instant invention is directed toward hydrogel electrode catheters for treatment and diagnosis of tissue. More specifically, the instant invention relates to treatment and diagnostic catheters with hydrogel virtual and sensing electrodes.
b. Background Art
Catheters have been in use for medical procedures for many years. Catheters can be used for medical procedures to examine, diagnose, and treat tissue while positioned at a specific location within the body that is otherwise inaccessible without more invasive procedures (e.g., medical procedures involving the human heart). During these procedures a catheter is inserted into a vessel located near the surface of a human body (e.g., an artery or vein in the leg, neck, or arm of the patient) and is guided or threaded through the vessels, sometimes with the aid of a guidewire or introducer, to a specific location within the body for examination, diagnosis, and treatment. For example, one procedure often referred to as “ablation” utilizes a catheter to convey energy (e.g., electrical or thermal) or a chemical to a selected location within the human body to create necrosis, which cuts off the path for stray or improper electrical signals. Another procedure often referred to as “mapping” utilizes a catheter with one or more sensing electrodes to monitor various forms of electrical activity in the human body.
It is well known that benefits may be gained by forming lesions in tissue during catheter ablation if the depth and location of the lesions being formed can be controlled. In particular, it can be desirable to elevate tissue temperature to around 50° C. until lesions are formed via coagulation necrosis, which changes the electrical properties of the tissue. When sufficiently deep lesions are formed at specific locations in cardiac tissue via coagulation necrosis, undesirable atrial fibrillations may be lessened or eliminated. “Sufficiently deep” lesions means transmural lesions in some cardiac applications.
Several difficulties may be encountered, however, when attempting to form adequately-deep lesions at specific locations using some existing ablation electrodes. For example, when forming lesions with radiofrequency (RF) energy, high temperature gradients are often encountered in the vicinity of the electrode. At the edges of some existing electrodes are regions of very high current density, leading to large temperature gradients and hot spots. These “edge effects” may result in the formation of undesirable coagulum and charring of the surface tissue. For example, undesirable coagulum may begin to form when blood reaches around 80° C. for an appreciable length of time, and undesirable tissue charring and desiccation may be seen when tissue reaches around 100° C. for an appreciable length of time. There are two main types of undesirable coagulum: coagulum that adheres to and damages the medical device; and coagulum blood clots or curds that may enter a patient's bloodstream, possibly resulting in other health problems for the patient. Charring of the surface tissue may also have deleterious effects on a patient.
During RF ablation, as the temperature of the electrode is increased, the contact time required to form an adequately-deep lesion decreases, but the likelihood of charring surface tissue and forming undesirable coagulum increases. As the temperature of the electrode is decreased, the contact time required to form an adequately-deep lesion increases, but the likelihood of charring surface tissue and forming undesirable coagulum decreases. It is, therefore, a balancing act trying to ensure that tissue temperatures are adequately high for long enough to create deep lesions, while still preventing or minimizing coagulum formation and/or charring of the surface tissue. Active temperature control may help, but the placement of thermocouples, for example, is tricky and setting the RF generator for a certain temperature becomes an empirical exercise as actual tissue temperatures are generally different from those recorded next to the electrode due to factors such as convection and catheter design.
Conventional mapping catheters may include, for example, a plurality of adjacent ring electrodes constructed from platinum or some other metal. Since mapping catheters are desirably disposable, incorporation of relatively expensive platinum electrodes may be disadvantageous.
Another difficulty encountered with existing ablation catheters and mapping catheters is how to ensure adequate tissue contact. For example, current techniques for creating linear lesions (the term “linear lesion” as used herein means an elongated, continuous or uninterrupted lesion, whether straight or curved and whether comprising a single line of ablation or a series of connected points or lines of ablation forming a track, that blocks electrical conduction) in endocardial applications may include dragging a conventional catheter on the tissue, using an array electrode, or using pre-formed electrodes. All of these devices comprise rigid electrodes that do not always conform to the tissue surface, especially when sharp gradients and undulations are present, such as at the ostium of the pulmonary vein in the left atrium and the isthmus of the right atrium. Consequently, continuous linear lesions are difficult to achieve. Whether forming lesions or mapping in a heart, the beating of the heart, especially if erratic or irregular, further complicates matters, making it difficult to keep adequate contact between electrodes and tissue for a sufficient length of time. For example, with a rigid electrode, it can be quite difficult to maintain sufficient contact pressure during lesion formation until an adequate lesion has been formed. These problems are exacerbated on contoured or trabeculated surfaces. If the contact between electrodes and tissue cannot be properly maintained, quality lesions or accurate mapping are unlikely to result.
Catheters based upon a virtual electrode that deliver RF energy via conductive fluid flowing into the patient's body address some of the difficulties with ablation catheters, but these ablation catheters often require high flow rates of the conductive fluid (e.g., typically around 70 milliliters per minute) to maintain effective cooling for high-power RF applications. The introduction of a large amount of conductive fluid into a patient's bloodstream may have detrimental effects on the patient.
Thus, there remains a need for ablation catheters and mapping catheters that address these issues with the existing designs.